The invention relates to a process for the production of high-grade gypsum and also of iron-oxide pigments from waste acid that accumulates in the course of the production of titanium dioxide in accordance with the sulfate process.
The utilisation or at least harmless elimination of waste acid is prescribed at sites in Europe and at most other sites for the production of titanium dioxide, so that over the course of time various processes have been developed with a view to utilisation: in printed publication EP-A 577 272 it is disclosed that a usable gypsum can be obtained from the waste acid by partial neutralisation with calcium carbonate. The metal-sulfate solution remaining after separation of this material, also designated as xe2x80x9cwhite gypsumxe2x80x9d, is then brought to a pH value of about 9 by addition of CaO or CaCO3, the material obtained in the process, also designated as xe2x80x9cred gypsumxe2x80x9d, having to be disposed of. In so doing, on the one hand the opportunity for recycling of valuable raw materials is lost, and on the other hand valuable landfill area is extensively used up, since, depending on the titanium raw material, 1 to 2.5 t of this waste accumulates per tonne of TiO2 pigment produced.
Another process, described in EP-A 0 133 505, for reprocessing the waste acid avoids these disadvantages by practically the entire amount of waste acid accumulating being re-used for the production of TiO2, the waste acid being firstly concentrated and, after separation of the filter salts that are precipitated out in the process, the 65-% to 85-% sulfuric acid being employed again for the purpose of decomposing ore. SO2 is obtained from the filter salts by thermal dissociation, and pure sulfuric acid or oleum, which is likewise re-used for the purpose of decomposing ore, is obtained from the SO2. Although this process minimises the consumption of raw materials, it is very energy-intensive and therefore costly.
A further process for reprocessing waste acid, described in printed publication U.S. Pat. No. 3,016,286, involves neutralisation of the waste acid and precipitation and separation of the hydroxides of Ti, Al, Cr and V, as well as subsequent precipitation of magnetite with ammonia. However, the disadvantages of this process are that, on the one hand, large quantities of ammonia are consumed for the purpose of neutralising the free sulfuric acid and that the magnetite which is precipitated out of the solution containing a considerable amount of ammonium sulfate displays no pigment properties.
In the case of a modification, described in printed publication EP-A 638 515, of the process in the form of an extraction of the magnesium from the solution containing ammonium sulfate, pure ammonium sulfate can subsequently be obtained by crystallisation and can be used as fertiliser. Although the proportion of usable material is increased by this means, the economy of the process is not satisfactory, by reason of the additional process steps and the inferior quality of the magnetite obtained from the solution containing a considerable amount of ammonium sulfate.
Another variant for reprocessing the waste acid consists, according to U.S. Pat. No. 4,137,292 and DE-A 24 56 320, in that gypsum and magnetite are precipitated out simultaneously by neutralisation of the waste acid with calcium compounds, whereby for utilisation of the two compounds a mechanical separation, for example by means of a hydrocyclone or by magnetic separation, has to be undertaken which, despite elaborate process steps, results neither in pure gypsum nor in a pure magnetite pigment. An optimisation of the process according to GB-A 1 421 773, to the effect that ammonium salts or alkali-metal salts are present simultaneously in the course of precipitation of the gypsum with calcium compounds, also does not avoid the aforementioned principal disadvantages of this process.
The object was therefore to develop a process that makes it possible for the waste acid accumulating during production of high-quality products to be made available as extensively as possible for meaningful utilisation.
Surprisingly it has been found that both a high-grade gypsum and high-grade iron-oxide pigments can be produced from the waste acid accumulating in the course of the production of titanium dioxide in accordance with the sulfate process if in a first step the waste acid is caused to react with a calcium compound to form gypsum and the latter is separated, optionally directly, from the remaining solution. In a second step, by increasing the pH value the elements Al, Ti, Cr, V and Fe(III) are at least partially precipitated out of the solution so obtained and are separated. In a third step the remaining solution containing iron(II) sulfate is caused to react with ammonia to yield a black iron-oxide pigment and subsequently the residual solution is separated. By addition of CaO or Ca(OH)2 the ammonia is liberated again from the solution containing ammonium sulfate that accumulates thereby and is recirculated into the process.
The invention provides a process for the production of gypsum and also of an iron-oxide pigment from the waste acid accumulating in the course of the production of titanium dioxide in accordance with the sulfate process, characterised in that in a first stage a partial neutralisation of the waste acid with a calcium compound is effected subject to precipitation and optionally direct separation of gypsum, subsequently the remaining solution is neutralised further in a second stage subject to precipitation of a deposit containing Ti, Al, Cr, V and optionally Fe(III), and from the solution containing iron-sulfate that is obtained after separation of the solids an iron-oxide pigment is precipitated in a third stage by addition of ammonia and also of an oxidising agent, ammonia being liberated by addition of CaO and/or Ca(OH)2 from the solution containing (NH4)2SO4 that is obtained after separation of the iron-oxide pigment and being recirculated into the process.
Separation of the gypsum obtained in the first stage may be effected directly after the precipitation, as a result of which a particularly high quality is obtained; however, separation of the gypsum may also be effected together with the solids precipitated in the second stage which contain Ti, Al, Cr, V and optionally Fe(III), as a result of which the process is simplified. Direct separation of the gypsum obtained in the first stage is preferred.
Partial neutralisation of the free sulfuric acid in the first stage is preferably effected by addition of calcium carbonate, calcium hydrogencarbonate, calcium oxide or calcium hydroxide or alternatively of other alkaline-reacting substances that contain one or more of the cited compounds, for example dolomite. However, partial neutralisation of the free sulfuric acid in the first stage may also be effected by means of a combination of individual substances cited above. Use is preferably made of calcium carbonate, calcium oxide or calcium hydroxide as calcium source. The use of finely ground lime (calcium carbonate) is particularly preferred, since the CO2 arising in the process can also be utilised.
It is furthermore possible, simultaneously with the neutralisation of the waste acid, to raise the iron content with a view to increasing the amount of iron-oxide pigment through neutralisation of the free sulfuric acid being undertaken in part with substances that contain metallic iron or iron(II), for example with scrap iron, mill scale, turnings or cast-iron filings. Further neutralisation of the free sulfuric acid can then be effected with calcium carbonate.
It is often expedient to dilute the waste acid prior to the reaction with the calcium compound, in order to reduce the viscosity of the reaction mixture. For the purpose of dilution, use may be made either of fresh water or alternatively of a process water that accumulates in the further course of the process. Dilution is preferably effected in a 1:1 ratio (parts by weight of waste acid to parts by weight of water). It is advantageous in this connection to slurry the Ca compound that is used for the purpose of neutralisation with a part of the dilution water and to neutralise the waste acid with this suspension.
Partial neutralisation of the waste acid in the first stage by addition of the calcium compound is preferably effected until a pH value from 1.0 to 3.0 is attained. At higher pH values the degree of whiteness of the gypsum can be impaired by coprecipitation of coloured contaminants, for example Fe(III) or other subgroup compounds. In particularly preferred manner neutralisation is effected until a pH value from 1.4 to 2.0 is attained, since the gypsum precipitated in this way contains particularly small amounts of coloured contaminants and particularly small amounts of TiO2, so that it also meets the requirements for special applications, for gypsum plaster for example. The pure calcium sulfate obtained in this way is separated from the liquid phase and is preferably washed with a view to removing coloured constituents of the solution.
Alternatively, for the partial neutralisation of the free sulfuric acid in the first stage, instead of the calcium compound a corresponding barium compound may also be chosen if barium sulfate is desired by way of product.
Subsequently the solution is neutralised further in the second stage, preferably until a pH value from 3.0 to 5.0 is attained, in particular until a pH value from 3.5 to 4.8 is attained. In this process, in particular in the course of the ensuing precipitation of magnetite, inconvenient titanium compounds and also, partially, Al, Cr, V and Fe(III) compounds are precipitated out. The deposit obtained in this way is separated from the liquid phase and may be either disposed of or used as raw material for the extraction of Ti, Al, Cr or V.
Prior to the precipitation of the deposit containing Ti, Al, Cr and V, Fe(III) which is optionally present is preferably reduced to Fe(II) by addition of a reducing agent, in particular metallic iron. As source of the metallic iron, substances resulting from industrial processes come into consideration in particular, such as turnings, cast-iron filings, stamped-metal waste or even mill scale. These substances are available in large quantities at low cost. As a result, the amount of the deposit resulting from the second precipitation stage to be disposed of or utilised further is reduced and the yield of high-grade iron-oxide pigment in the following stage is increased. This reduction of Fe(III) may be effected either prior to the precipitation of gypsum in the first stage or alternatively afterwardsxe2x80x94ie, immediately prior to the precipitation of the trivalent and tetravalent metallic ions. In the course of the reduction of the Fe(III), however, the reaction should be controlled in such a way that as little as possible Ti3+ arises which would only be precipitated out incompletely in the second stage. Precipitation of the titanium in the second stage can optionally also be improved by addition of seeds consisting of titanium oxide hydrate.
For this second neutralisation, by way of neutralising agent use may be made of a compound that forms no sparingly soluble sulfates, so that the accumulation of solids is minimised. Alternatively, however, by way of neutralising agent use may also be made of a compound that forms readily filterable, sparingly soluble sulfates and therefore clearly improves the filterability of the entire quantity of solids accumulating. Finally, it is also possible for a combination of neutralising agents, one of which forms soluble sulfates and another of which forms sparingly soluble sulfates, to find application with a view to selective optimisation of this process step.
Suitable by way of neutralising agents are compounds from the group comprising gaseous NH3, NH3 dissolved in water, oxides, hydroxides, carbonates and hydrogen-carbonates of the alkali metals or alkaline-earth metals, as well as alkaline-reacting mixtures that contain at least one of the cited compounds. For reasons of economy the use of alkaline-reacting ashes as neutralising agent is particularly advantageous, for example ashes resulting from the combustion of coal or from the incineration of refuse. By way of neutralising agent in the second stage, gaseous ammonia or ammonia dissolved in water is preferably employed predominantly or exclusively. Particularly preferred is the simultaneous use of ammonia and alkaline-reacting ash. In this connection use is preferably made of ammonia and of ash only in a small proportion, in particular  greater than 90% alkali equivalents of ammonia and  less than 10% alkali equivalents of ash. With this manner of proceeding the small proportion of ash brings about a clear improvement in the filterability of the metal hydroxides, whereas the quantity of solids accumulating is not increased significantly. Instead of ash, use may also be made of an alkaline calcium compound, for example CaO, but as a rule the use of ash is more attractive economically.
The solution containing iron sulfate that is obtained after the second neutralisation stage and separation of the solids is adjusted for the precipitation of the iron-oxide pigment preferably to a concentration of 150 to 250 g, in particular 180 to 190 g, FeSO4 per liter. This concentration can optionally be achieved by evaporation. The adjusted solution containing iron sulfate is converted in known manner into an iron-oxide pigment, preferably a magnetite pigment, by addition of ammonia and an oxidising agent, preferably by blowing in oxygen or gases containing oxygen, in particular air, (Ullmann""s Encyclopaedia of Industrial Chemistry, 5th Edn., Vol. A 20, p 297 et seq).
For the preferred production according to the invention of a black iron-oxide pigment, preferably such a quantity of gaseous ammonia or ammonia dissolved in water, in particular 25-% ammonia solution is added to the solution containing iron sulfate that the ratio of iron(II) ions to ammonia amounts to 0.4 to 0.65, in particular 0.5 to 0.58, equivalents. The calculated quantity of ammonia is preferably added to the solution containing iron sulfate at a temperature between 60 and 95xc2x0 C., in particular between 75 and 95xc2x0 C. It is also possible to submit an aqueous ammonia solution.
Oxidation is subsequently effected with an oxidising agent. By way of oxidising agent use may be made, for example, of compounds from the group comprising oxygen, ozone, H2O2, sodium hypochlorite, sodium-hypochlorite solution, perchlorates, nitrates and chlorine. Oxygen or a gas mixture containing oxygen, in particular air, is preferably introduced into the reaction mixture. The oxidation is completed as soon as the Fe(II) content of the suspension is less than 1 mole -%.
Reprocessing of the pigment suspensions is effected by means of the known steps of filtration, drying and grinding. A person skilled in the art will be able, by suitable variation of the production conditions, to produce a broad palette of iron-oxide pigments of various particle sizes and consequently of various tones and stabilities.
By annealing the black iron-oxide pigment obtained it is possible to produce high-grade brown iron-oxide pigments. In this connection the tone of the brown iron-oxide pigments can be fixed within certain limits by varying the annealing conditions.
From the solution containing ammonium sulfate remaining after the precipitation and separation of the black iron-oxide pigment the ammonia is recovered by addition of CaO and/or Ca(OH)2 and is recirculated into the process. After the addition of CaO and/or Ca(OH)2 until a pH value of  greater than 10 is attained, ammonia escapes in gaseous form and can be employed again directly without compression or other treatment in connection with the precipitation of the metal hydroxides in the second stage or in connection with the precipitation of magnetite in the third stage. From the suspension from which the gaseous ammonia was obtained after the addition of the CaO and/or Ca(OH)2 the residual ammonia content is preferably removed by stripping with steam or air in a stripping column. The ammonia solution obtained in this way (about 25-%) can be employed again without further treatment in connection with the precipitation of the metal hydroxides in the second stage or in connection with the precipitation of magnetite in the third stage. The remaining deposit containing gypsum can be filtered off and utilised as building material after neutralisation, for example after washing with dilute sulfuric acid. The alkaline filtrate substantially contains magnesium sulfate and ammonium sulfate and can be used as fertiliser or, after neutralisation, disposed of in the form of waste water containing neutral salt.
In this way both a high-grade gypsum and a high-grade iron-oxide pigment, each in pure form, are obtained from the waste acid, and the resulting waste is minimised.
Particularly advantageous in the case of the three-stage process with the separate separation of the gypsum from the first stage is the fact that the gypsum obtained in accordance with the invention contains particularly small amounts of colouring contaminants such as are constituted by compounds of the elements Fe, Cr, V, for example. Furthermore, the gypsum contains particularly small amounts of Ti compounds, making it suitable also for the production of high-grade gypsum plasters or for the production of anhydrite. The iron-oxide pigment obtained in accordance with the invention is not contaminated by gypsum and exhibits particularly good optical pigment properties, since the parent solution containing iron sulfate which is used for this purpose on the one hand contains particularly small amounts of titanium salts and, on the other hand, by reason of the precipitation of gypsum in the first stage does not have a very high concentration of neutral salts such as, for example, Na2SO4, K2SO4 or (NH4)2SO4. Furthermore, by reason of the preliminary precipitation of the metal hydroxides in the second stage the iron-oxide pigment produced in accordance with the invention is low in heavy-metal compounds, particularly in compounds of Cr and V. By virtue of the process according to the invention it is possible to process the waste acid that accumulates as waste with simple process-engineering operations and with a comparatively low energy demand to yield high-grade products in the form of pure gypsum, iron oxide having pigment properties and pure CO2 and to minimise the waste. The amount of waste from 7 to 8 t waste acid per t TiO2 is preferably reduced to only 0.2 to 0.7 t filter cake (from the second stage) per t of TiO2 produced. In addition, the process according to the invention is particularly economical, since the ammonia is recovered and therefore does not pollute either the metal hydroxides to be disposed of or the waste-water load.
If the separation of the gypsum that is precipitated out in the first stage is effected jointly with the separation of the metal hydroxides that are precipitated out in the second stage and of the gypsum that is additionally precipitated out in the second stage, no residues of any kind any longer accumulate that have to be disposed of or reprocessed (see Example 6).
The iron-oxide pigments obtained in accordance with the invention can be used for the dyeing of paints, lacquers, plastics, building materials, paper and other materials. The magnetite obtained in accordance with the invention may furthermore also find application as magnetic pigment for toner.
The tone of the iron-oxide pigment obtained is determined in accordance with the following directions:
Measurement of the Mass Tone of Iron-oxide Pigments
The pigment is dispersed with a muller (plate-type automatic muller) in an air-drying lacquer system. The lacquer system (lacquer) consists of the following components:
The components are intermixed with a high-speed stirrer to produce the finished lacquer. Use is made of a plate-type automatic muller as described in DIN EN ISO 8780-5 (April 1995). An (copyright)ENGELSMANN JEL 25/53 muller with an effective plate diameter of 24 cm is used. The speed of rotation of the lower plate amounts to about 75 minxe2x88x921. As a result of hanging a 2.5 kg loading weight on the loading frame the force between the plates is adjusted to about 0.5 kN. 0.8 g pigment and 2.00 g lacquer are dispersed in a stage at 100 revs with a 2.5 kg loading weight in accordance with the process described in DIN EN ISO 8780-5 (April 1995) Section 8.1. The muller is opened and the lacquer is swiftly collected on the lower plate outside the midpoint. Then a further 2.00 g lacquer are added and the plates are closed together. After two stages at 50 revs without loading weight the preparation is completed.
The pigmented lacquer is spread with a film spreader (gap height at least 150 xcexcm, at most 250 xcexcm) on a non-absorbent cardboard. The lacquered cardboard (coating) is then dried for at least 12 h at room temperature. Prior to the colour measurement the coating is dried for one hour at about 65xc2x0 C. (xc2x15xc2x0 C.) and cooled.
Measurement of the Tint Tone of Iron-oxide Pigments
The pigment and the brightening agent are dispersed with a muller (plate-type automatic muller) in an air-drying lacquer system. By way of brightening agent use is made of a commercially available (copyright)Bayertitan R-KB-2 titanium-dioxide pigment (Bayer AG). This pigment corresponds to Type R 2 in ISO 591-1977. The lacquer system (lacquer) corresponds to that for determining the mass tone (see above).
The components of the lacquer system are intermixed with a high-speed stirrer to produce the finished lacquer.
The pigmented lacquer and the lacquer coating are produced in the manner described in connection with the determination of the mass tone (see above), 0.1500 g pigment to be tested, 0.7500 g Bayertitan R-KB-2 and 2.00 g lacquer being weighed in.
Measurement of the Mass Tone of Gypsum on Powder Compact
In order to produce the powder compact, 10 g of the gypsum sample were compacted linearly from 0 to 120 bar within 30 seconds in a hydraulic press-and were maintained at 120 bar for 6 seconds. Colorimetric characterisation of the powder compact is effected in a manner analogous to the colorimetric characterisation of lacquer coatings.
Colour-measuring Instrument
Use is made of a spectrophotometer (xe2x80x9ccolour-measuring instrumentxe2x80x9d) having an Ulbricht sphere with measurement geometry d/8 without gloss trap. This measurement geometry is described in ISO 7724/2-1984(E) point 4.1.1, in DIN 5033 Part 7 (July 1983) point 3.2.4 and in DIN 53 236 (January 1983) point 7.1.1. Use is made of a (copyright)Dataflash 2000 measuring instrument available from Datacolor International.
The colour-measuring instrument is calibrated against a white, ceramic working standard as described in ISO 7724/2-1984 (E) point 8.3. The reflection data of the working standard compared to an ideally dull-white body are stored in the colour-measuring instrument, so that after calibration with the white working standard all colour measurements are related to the ideally dull-white body. Calibration of the black point is carried out with a black hollow body available from the manufacturer of the colour-measuring instrument.
Colorimetry
Any gloss trap which may be present is disconnected. The temperature of the colour-measuring instrument and of the test piece amounts to about 25xc2x0 C. xc2x15xc2x0 C.
The lacquer coating is placed onto the colour-measuring instrument in such a way that the measuring hole is covered by a central point of the layer of lacquer. The entire coating has to lie flat. The measuring hole has to be totally covered by the layer of lacquer. The measurement is then carried out.
Calculation of the CEE Coordinates
From the measured reflection spectrum the CIE coordinates L*, a* and b* of 1976 are calculated in accordance with the calculating instructions given in ASTM E 308-1985, point 7. Use is made of the weighting functions of the standard illuminant C and of the 2xc2x0 standard colorimetric observer of 1931 given in ASTM E 308-1985, Table 5.6. The wavelength range is between 400 nm and 700 nm. The wavelength interval amounts to 20 nm. No gloss is subtracted in the calculation. The reflectance values obtained are converted in accordance with DIN 5033, Part 3 (July 1992) into the values pertaining to the CIELAB colour data system.
The relative colour intensity is calculated by analogy with the relative scattering power according to DIN 53 165 (point 3.4) with Bayertitan R-KB-2 as brightening agent and with a suitable Bayferrox reference pigment (instead of carbon black); by way of p∞ use is made of the tristimulus value Y/100.